Oddly Wired
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ownership

You value your possessions more than they are worth.

People demand a higher price to sell an item they own than they would pay to acquire that exact same item.

This is the endowment effect. Once an object becomes part of your 'self,' it gains subjective value that far exceeds its market price. The psychological pain of losing the object outweighs the joy of acquiring it.

Ownership changes the math of your preferences.

Where this comes from

Study
Kahneman, Knetsch, and Thaler, 1990
What the check found
The definition and attribution are accurate to the seminal 1990 paper by Kahneman, Knetsch, and Thaler, even though the provided reference material does not contain a quote verifying the claim.
Confidence
Solid, with limits

Related

One of these every day, and every one names the study it came from. When a finding is contested, it says so.